The invention relates to data compression. More specifically, the invention relates to data coding and embedded bitstreams.
Data compression is often used for reducing the cost of storing large data files on computers as well as reducing the time for transmitting large data files between computers. In the so-called xe2x80x9ctransform methodsxe2x80x9d data is transformed into coefficients that represent the data in a frequency domain. Coefficients may be quantized (lossy compression) without significantly affecting the quality of data that is eventually reconstructed from the quantized coefficients. Redundancy in the coefficients may then be reduced or eliminated (lossless compression) without affecting quality of the reconstructed data.
One well known class of transforms are Wavelet transforms. The Wavelet transforms may be used to perform subband decomposition and produce coefficients that describe the data in a hierarchical multiscale representation. Wavelet transforms have proven useful for the compression of images and the analysis of signals. They have been proposed as the transforms for the emerging xe2x80x9cJPEG-2000xe2x80x9d standard.
Among the advantages of the Wavelet transforms, the transform coefficients can be ordered in a hierarchical structure and transmitted in an xe2x80x9cembedded bitstream.xe2x80x9d The embedded bitstream has a property whereby prefixes of the bitstream yield a continuum of lower rate descriptions of the data at the highest possible levels of quality. If the embedded bitstream is truncated during transmission of image data, for instance, the information already transmitted allows an entire image to be reconstructed. The quality of the reconstructed image is dependent upon the amount information transmitted. If an embedded bitstream is truncated, a complete image of reduced quality can be reconstructed from the transmitted bits. In contrast, truncation of a non-embedded transmission might only allow several rows of an image to be reconstructed.
As additional information is transmitted, the quality of the reconstructed image is improved. If the entire bitstream is transmitted without truncation, a lossless or near-lossless image can be reconstructed.
The transmission just described is often referred to as a xe2x80x9cprogressive-by-qualityxe2x80x9d image transmission. The coefficients are described by bit-planes, and the most significant coefficient bits (that is, the coefficient bits conveying the most important information) are transmitted first.
Another type of transmission is often referred to as a xe2x80x9cprogressive-by-resolutionxe2x80x9d transmission. The progressive-by-resolution transmission involves ordering the coefficients according to different levels of image resolution. The different levels are identified by markers in the embedded bitstream. A computer may use the markers to parse the bitstream and transmit the data for the coefficients corresponding to a resolution that is specified by the receiving computer. The receiving computer can reconstruct an image according to the specified resolution.
Versatility is very desirable for systems that compress and reconstruct images. The following features for such systems are very desirable: scalability, random access, rate control and resolution control. Perhaps even more desirable is a system that can combine all of these features.
According to one aspect of the present invention, a subband decomposition can be processed by partitioning each subband into a plurality of blocks; and encoding the blocks of each subband, wherein a block bitstream is generated for each block. The blocks of each subband are coded independently of each other.
Such processing allows a layered embedded bitstream to be generated with greater scalability, random access, error resilience and coding efficiency.
It also allows for efficient rate control, low complexity, low memory and region-of-interest (xe2x80x9cROIxe2x80x9d) reconstruction. The present invention can be used to provide better support for client-server applications and greater flexibility for proprietary encoders to optimize the bitstream by exploiting xe2x80x9cpsycho-visualxe2x80x9d properties without making any changes to the decoder or bitstream syntax.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.